My experience is breast forms and push ups bra do not work well.
Push up bras are designed to push the breast tissue inwards and upwards to enhance a lack of cleavage. Natural breast tissue will form when pushed like that, silicone breast forms don’t, they’re too stiff. What happens is the forms get pushed up but don’t change shape and just look like a breast form in the wrong place. I may be wrong and there are breast forms that will work, but since the idea of push ups is to enhance and maximise what a GG hasn’t got in the first place I would just get the right size, larger form in a good underwire bra. I find underwires (which GGs seem to hate) work well because they give the form a little bit of shape and something to sit in, if you get me.
The opposite of push-ups are minimiser bras for very large breasted GGs, they do the opposite, they try to make the breasts look smaller than they actually are. Again, pointless for us as our forms should be giving us perfect, perky breasts. If you want to show more cleavage then maybe get a size larger?
Hope that helps!